The present invention relates to the sealing of anodized aluminum and aluminum alloy structures, and more particularly to the sealing of such structures to achieve enhanced resistivity and dielectric properties.
Hardcoat anodization of aluminum and aluminum alloys is an electrolytic process which is used to produce thick oxide coatings with substantial hardness. Such coatings are to be distinguished from natural films of oxide which are normally present on aluminum surfaces and from thin, electrolytically formed barrier coatings.
The anodization of aluminum to form thick dielectric coatings takes place in an electrolytic bath containing an oxide, such as sulfuric or oxalic acid in which aluminum oxide is slightly soluble. The production techniques, properties, and applications of these aluminum oxide coatings are described in detail in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Robert Draper Ltd., Peddington, England (chapter IX page 563). Such coatings are extremely hard and mechanically superior to uncoated aluminum. However, the coatings contain pores in the form of fine tubes with a porosity on the order of 10.sup.10 to 10.sup.12 pores per square inch. Typical porosities range from 10 to 30 percent by volume. These pores extend through the coating to a very thin barrier layer of aluminum oxide, typically 300 to 800 Angstroms.
For improved mechanical properties as well as to prevent staining, it is customary practice to seal the pores. One standard sealing technique involves partially hydrating the oxide through immersion in boiling water, usually containing certain nickel salts, which form an expanded boehmite structure at the mouths of the pores. Oxide sealing in this manner will not support an electrostatic charge due to the ionic conductivity of moisture trapped in the pores.
Another method of sealing an anodized aluminum member is disclosed by Quaintance in U.S. Pat. No. 3,715,211. This is a method of cold sealing by the photopolymerization of an organic liquid applied to the anodized surface.
U.S. Pat. No. 3,615,405 discloses a method of fabricating an electrophotographic oxide surface by means of impregnating the porous oxide surface of an aluminum article with an "imaging material." The process creates a member with direct contact between the imaging material and the conductive substrate over which the porous oxide layer is formed. This patent does not disclose a step of dehydrating the oxide pores prior to impregnation with an imaging material (the article is placed in a vacuum oven only after coating with an impregnant material). As such, there is a likelihood of trapped moisture, which would be deleterious to the dielectric properties of the impregnated anodic layer. In order to provide discharge in radiation struck areas, U.S. Pat. No. 3,615,405 requires contact of the "electrographic imaging material" with the conducting substrate. In the present invention, the sealing material contacts an insulating barrier layer.
These foregoing references cannot be used for the processing of an aluminum cylinder for use in electrostatic imaging with pressure fusing and transfer as herein disclosed. In such a system, an electrostatic charge is placed on the insulating surface of the cylinder. This image is then toned as disclosed, for example, in U.S. Pat. No. 3,662,395, and the toned image transferred to plain paper. Table 2 of that patent indicates that a porous aluminum oxide surface sealed with teflon is not satisfactory for electrostatic imaging due to the low breakdown voltage and low pore insulation resistance of the aluminum oxide surface. The organic resin sealant fails to achieve the necessary high abrasion resistance and coating hardness.
A drum coated with an insulating film capable of supporting an electrostatic charge is disclosed in U.S. Pat. No. 3,907,560. The dielectric surface is a barrier layer aluminum oxide film since it is stated that the porous anodized aluminum oxide layer functions as a conductor rather than a dielectric. Although a barrier layer anodized aluminum film is a good insulator, being non-porous, the maximum thickness of barrier layer films is restricted to the region of at most 1/2 to 1 microns. At this thickness, the maximum voltage the layer will support is limited and the surface is not hard in a conventional sense since any localized strains are transmitted through the thin film with subsequent deformation of the aluminum substrate.
The limitations of the thin barrier film are overcome in U.S. Pat. Nos. 3,937,571 and 3,940,270 by the use of a duplex anodized aluminum coating. The coating is prepared by electrolytically oxidizing an aluminum surface and thereafter continuing the electrolytic oxidization under conditions which produce a barrier aluminum oxide layer. Not only does this increase the complexity of fabricating the anodized layer, but the limiting thickness is approximately 20 microns and the surface potential to which the oxide layer may be charged is a maximum of 620 volts.
U.S. Pat. No. 1,978,112 discloses a technique for forming a lubricant surface layer on aluminum articles for example to prevent seizing of adjacent aluminum machine parts. The lubricant material is a metal stearate such as zinc stearate which is applied by immersing the article in a molten bath. A preferred embodiment of this technique entails an initial anodizing step to provide enhanced adhesion of the stearate. This patent is directed to providing a surface layer of stearate, and is inconsistent with the goals of providing a smooth, hard dielectric surface. In fact, the skilled artisan following the teachings of '112 would not produce a surface having favorable charge acceptance properties, even if subsequent steps were taken to remove the surface stearate, due to inadequate impregnation of the pores.
U.S. Pat. No. 3,664,300 discloses electrostatic imaging apparatus incorporating a photoconductive or dielectric imaging cylinder on which a latent electrostatic image is formed, toned, and transferred using an electrostatic assist. The apparatus includes a post-transfer station in which a solid hydrophobic metal salt of a fatty acid is applied to the surface via a fibrous applicator. This treatment allegedly reduces surface wear, while providing enhanced electrostatic toner transfer. However, the technique is not directed to achieving increased charge acceptance, but rather is an ongoing treatment which will have limited effect in this regard. Furthermore, the technique is inconsistant with the requirements of essentially complete pressure toner transfer.
Commonly assigned U.S. patent application Ser. No. 072,524, which is a continuation-in-part of application Ser. No. 822,865, now abandoned, discloses a method for forming a dielectric surface layer involving the preliminary dehydration of an anodized aluminum member followed by impregnation of surface apertures of the dehydrated member with an organic dielectric material. The preliminary dehydration may be accomplished by heating the anodized member in a vacuum or in air, or alternatively by storing it in a desiccant dry box. This application discloses a class of impregnant materials broadly described as organic resins. The method disclosed therein has been found effective to fabricate a dielectric surface with improved resistivity, dielectric properties, and toner release properties. It has been observed, however, that the dielectric properties are deleteriously affected by elevated humidities. Because these materials are usually applied at room temperature, special measures must be taken to control the environment during impregnation to minimize the risk of dehydration. Furthermore, it can be difficult to remedy the problem of an initially uneven application of the impregnant material.
Accordingly, it is a primary object of this invention to provide desired dielectric properties in the treatment of members of porous anodized aluminum and aluminum-based alloys. A related object is to improve the dielectric strength and increase the resistivity of such members. Another related object is the achievement of thick dielectric surface layers with a high voltage acceptance and low charge decay rates.
A further object of the invention is the achievement of a treated aluminum surface that will yield essentially total pressure transfer of a toned electrostatic image to plain paper and other substrates.
Yet another object of the invention is the achievement of a surface which maintains the above properties at elevated humidities.
Still another object of the invention is that the fabrication technique be easily implementable. As a related object, the technique should allow simple remedial steps to meet the above criteria where the initial fabrication is unsuccessful.